1. Field of the Invention
The present invention relates to an exposure method and a apparatus that employs a charged particle beam, such as an electron beam, and in particular to an exposure method and apparatus whereby beam drift, due to the deposit in an apparatus of a contaminating substance, or substances, can be prevented.
2. Related Arts
As a result of the high level integration required for an integrated circuit, there is a need for additional development of micromachining techniques. One of the current micromachining techniques used involves exposing a wafer or a reticle mask by irradiating it with a charged particle beam, such as an electron beam. But, in order to respond to a future need for super-micromachining techniques, it may become necessary to expose a wafer directly by the beam.
Although the present invention can be widely applied for wafer exposure using a charged particle beam, the term "electron beam" will be used hereinafter instead of "charged particle beam" to simplify the explanation. In an electron beam exposure apparatus, electrons are produced by an electron gun and are accelerated by an electric field so as to generate an erectron beam. The shape and the direction of the electron beam are controlled by an electromagnetic lens and a deflector, both of which are provided in a lens barrel. Normally, an electron beam is shaped and given a rectangular cross section by being passed through a first slit that has a specified rectangular shape. The electron beam is then formed and given an exposure pattern cross section by being passed through a second slit, or a blanking aperture array (BAA) mask (a transmission mask, as a general term) having a predetermined mask pattern shapes While the half angle of convergence of the electron beam, which now has the exposure pattern cross section, is restricted by a round aperture, or diaphragm, the electron beam is passed through a projection lens and a deflector at the final stage, and is irradiated to a sample, such as a wafer or a reticle mask. For electron beam exposure, it is known that micromachining of an area of about 0.05 .mu.m or smaller can be performed with a positioning accuracy of 0.02 .mu.m or less.
However, it is also known that the positioning accuracy of an electron beam is progressively degraded as time elapses. The primary factor contributing to the degradation of accuracy is a positioning shift of the electron beam, commonly known as "beam drift". One of the primary causes of beam drift is a charge-up drift caused by contamination at an electrostatic deflecting electrode in the vicinity of a projection lens or in the lower portion of a lens barrel. Another cause is a charge-up drift that occurs upstream from the projection lens.
FIG. 9 is a schematic diagram illustrating a projection lens in an exposure apparatus. A main chamber 4 is used to store a wafer W, as a sample, and a vacuum is maintained therein by a turbo molecular pump P3. A portion 71 is in atmospheric pressure, and stores a projection lens 32, which is an electromagnetic lens for example, and a main deflector 33. Further, a sub-deflector 34 and its cover 70 are provided in an evacuated mirror column. An electron beam EB is irradiated to desired locations on the surface of the wafer W by the projection lens 32 and the main and sub-deflectors 33 and 34.
The surface of the wafer W is normally coated with a resist film composed of organic material. When the wafer W is irradiated with a high energy electron beam, a gas is generated from the organic material. The gas generated from the organic material either attaches to the surface of the cover 70 and the surfaces of other components, or a carbon element in the gas is evaporated by reflected electrons. As a result, a highly insulating contaminant CON is generated on the surface of the cover 70. When charges, such as reflected electrons and secondary electrons, are accumulated in the contamination CON, an electric field is produced around the contamination. The electric field causes the position shift of the electron beam irradiated from above.
Contamination may also occur at the previously described round aperture, or diaphragm, etc., which is also provided upstream from the projection lens 32, in the same manner as described, affecting an electric field nearby.
The fluctuation of the distribution of the electric field causes the lateral drift of an electron beam and the defocusing of a beam.
These contaminants are accumulated over a long period of time. FIG. 10 is a graph showing the tendency of a change in a beam drift that occurs due to the contaminations. The horizontal axis represents time in months, and the vertical axis represents beam drift distance. As is shown by this graph, the beam drift distance tends to increase gradually over a period of several months. In the example in FIG. 10, since an exposure apparatus is cleaned every three months, the beam drift distance immediately after cleaning is 0. When almost three months have elapsed, however, the drift distance reaches 0.04 .mu.m, which is too large for an electron beam exposure apparatus that performs micromachining.
As a method for removing such contamination, the present applicant proposed a method for cleaning all components of an exposure apparatus by introducing oxygen into the apparatus and inducing plasma excitation (e.g., Japanese Patent Application No. Hei 5-138755, U.S. Pat. No. 5,401,974). When utilizing a cleaning method using plasma etching, however, the cleaning is not performed until a drift value has reached a specific level, and therefore the drift occuring up to that time can not be avoided. If the apparatus is cleaned frequently, so as to reduce the drift distance as much as possible, the availability factor for the electron beam exposure apparatus is reduced. Further, since the generation of a high frequency current accompanies plasma excitation, the metal plated on a ceramic portion of an electrode or a barrel is sputtered. As a result, these components must be replaced after a certain number of cleanings.